Digital printing or copying machines work, for example, by the electrophotographic process, in which a latent electrostatic image is developed by charged toner particles. These are transferred to an image receiver substrate, hereinafter, the substrate. Thereafter, the image that has been developed and transferred to the substrate is fixed by heating and melting the toner particles. Contact methods are often used to melt the toner particles, during which the toner particles are brought into touching contact with corresponding devices, such as hot rolls or rollers. The drawback is that the use of silicone oil as separating agent is generally required to prevent a sticking of the melted toner to the heating device. Furthermore, the construction, the maintenance, and the operating costs of these touch type heating devices are substantial and, thus, cost intensive. Moreover, the error rate produced by the contact heating devices is relatively high. Furthermore, noncontact heating devices and methods are known for fixing the toner that has been transferred to paper (for example), in which the toner particles are melted, for example, by heat from thermal/microwave radiation or by hot air.
The contact and the noncontact melting methods use, for example, a toner whose glass transition temperature (TG) lies in a range of 45° C. to 75° C. The glass transition temperature, in which the toner starting from the solid state begins to become soft, can be influenced by the choice of the raw materials and by adding certain additives to the toner. In a fixation device having at least one heating device for the toner, both the toner and the substrate itself is heated. In order to make sure of a good fixation of the toner on the substrate, the surface temperature of the substrate must be in the region of the glass transition temperature of the toner or higher. The toner will reach or exceed the glass transition temperature (TG) already in the region of the heating device.
There are familiar printing and copying machines in which the substrate is printed or coated on both sides, and either the very same image generating and transferring device and heating device are used for the printing on the front and back side, or a separate image generating and transferring device and heating device are used. In order to fix the toner image, the substrate is frequently taken by a transport belt, on which the substrate is placed, past the at least one image generating and transferring device and the corresponding heating device. In this case, a first toner image is initially transferred to a first substrate side and then fixed. After this, a second toner image is transferred and fixed on the second substrate side. Therefore, when melting the second toner image, the first substrate side with the already fixed first toner image located on it lies on the transport belt. The disadvantage here is that, during the melting of the second toner image, the first toner image can become heated to such an extent that it becomes soft and has a tendency to stick to the transport belt. This can lead to several undesirable effects: due to the sticking, a substrate jam can occur when taking the substrate from the transport belt to a subsequent part of the machine. Furthermore, the appearance of the toner image can change in the areas where it sticks to the transport belt. This results in problems of image quality, for example, the toner image has an irregular gloss.